Among color display devices used for image display on a computer screen or television, a display device using a plasma display panel (hereinafter referred to as a “PDP”) has recently been drawing attention, as a large, thin, and lightweight color display device.
A plasma display device using a PDP performs additive color mixing of so-called three primary colors (red, green, and blue) to provide full-color display. For the full-color display, a plasma display device has phosphor layers for emitting the respective three primary colors, i.e. red (R), green (G), and blue (B). Phosphor particles constituting these phosphor layers are exited by ultraviolet light generated in discharge cells of the PDP to generate visible light of respective colors.
Known as compounds used for phosphors are (YGd)BO3:Eu3+ and Y2O3:Eu3+ for red emission, Zn2SiO4:Mn2+ for green emission, and BaMgAl10O17:Eu2+ for blue emission. Each of these phosphors is fabricated by mixing specific materials and then firing the mixture at high temperatures of at least 1,000° C. for solid-phase reaction (see “Phosphor Handbook” p.219 and 225, Ohmsha, for example). The phosphor particles obtained by this firing are used after they are milled and classified (average diameter of red and green particles: 2 to 5 μm, average diameter of blue particles: 3 to 10 μm). The phosphor particles are milled and classified for the following reason. In general, when phosphor layers are formed on a PDP, a technique of screen-printing a paste of phosphor particles of each color is used. In application of the paste, the smaller and more uniform diameters of phosphor particles (i.e. a uniform particle size distribution) can easily provide the smoother coated surface. In other words, when phosphor particles have smaller and more uniform diameters and shapes approximating to a sphere, the coated surface is smoother. The smoother coated surface increases the packing density of the phosphor particles in a phosphor layer and the emission surface area of the particles, thus increasing the luminance of the plasma display device.
However, the smaller diameters of phosphor particles increase the specific surface area of the phosphor and thus defects on the surface of the phosphor. For this reason, a large quantity of water, carbonic acid gas, or hydrocarbon-containing organic substances are prone to adhere to the surface of the phosphor. Especially for a blue phosphor made of Ba1−xMgAl10O17:Eux or Ba1−x−ySryMgAl10O17:Eux, crystal structures thereof has layer structures each made of three layers, i.e. BaO, 4Al2O3, and MgAl2O4 (where Eu and Sr substitute for part of Ba). It is known that, among the layers, there is oxygen (O) vacancy in layers containing Ba atoms (Ba-O layers) (see “Display and Imaging”, 1999, vol. 7, pp 225-234 and “OYO BUTSURI (Applied Physics)”, vol. 70, No.3, 2001, pp310, for example). For this reason, water existing in air is selectively adsorbed onto the surface of such a Ba-O layer. As a result, because a large quantity of water is released onto a panel in a panel manufacturing process, the water reacts with the phosphor and MgO during discharge. This poses problems of luminance degradation and chromaticity shift (color shift and image bum caused by the chromaticity shift), or decrease in drive voltage margin and increase in discharge voltage.
On the other hand, disclosed method to address these problems is coating the entire surface of a phosphor with a crystal thin layer made of a material, such as Al2O3, in order to recover the defects in Ba—O layers (see Japanese Patent Unexamined Publication No. 2001-55567, for example). However, coating the entire surface of a phosphor poses another problem: the thin layer of coating absorbs ultraviolet light and thus decreases the emission luminance of the phosphor.
In order to address these problems, the present invention aims to inhibit water adsorption onto the surface of a blue phosphor, decrease luminance degradation and chromaticity shift of a phosphor, or improve discharge characteristics thereof.